The invention concerns a method for producing filter bellows from a zigzag folded filter medium, in particular for use as flat air filter in an internal combustion engine, in particular of a motor vehicle.
End face folds are the two outer folds of opposite end faces of the filter bellows of a filter element. End face rims are the two free rims of the filter medium of which the filter bellows is formed and which extend along the end face folds and delimit them at the end faces of the filter bellows. End edges of the filter bellows are the two other free rims of the filter bellows which extend between the end face rims and extend in accordance with the folding of the filter bellows. The fold edges are the edges along which the filter medium is folded. In a commercially available zigzag folded, approximately parallelepipedal filter bellows, the end face rims and the fold edges in general are straight and extend parallel to each other. Viewing the filter bellows from the side, the end edges extend in a zigzag shape and perpendicular to the end face rims and the fold edges. Prior to folding the filter medium, the end edges of the filter bellows, which later on exhibits an approximately parallelepipedal imaginary envelope, extend straight and parallel to each other. The imaginary envelope is defined by the end edges, the neighboring fold edges or end face rims at the clean fluid side of the filter bellows and the neighboring fold edges or end face rims at the raw fluid side.
In filter bellows of flat filter elements, the filter media are not closed, i.e., the end face folds like the end edges are not connected to each other. In contrast thereto, in filter bellows of round filter elements the filter media are closed, i.e., their end face folds are connected to each other. Filter bellows of flat filter elements can be planar but also can be provided with bends in different directions.
Rectangular or trapezoidal filter bellows for flat filter elements or filter bellows with cut-off corners that are produced from an endless running web of a filter medium are prior art. By laser cutting, for example, a plurality of geometries is conceivable which however partially entail a significant paper loss and thus waste of filter medium.